What I had to do (did I do it right?) was pretend that I wanted to change a button behavior to a new custom action. This allowed me to create the actionscript as an item that could then be shown in the menu. Next, I changed the button I'd changed back to it's default.

Is that the only way to do that? It seemed non-intuitive to go to mapping (which is about buttons) in order to create a menu entry.